This invention relates to a method and an apparatus therefor of automatic adjustment of biasing current for recording sound by three-head type tape-recorder which has a recorder head and a reproducer head as well as an eraser head, one separate from and independent of another, so that recording sound by the recorder head on a running magnetic tape and immediately reproducing the recorded sound by the reproducer head immediately downstream of the recorder head in the sense of the tape-running may be made.
In this field of art, it is a conventionally established knowledge that the reproduced sound output level shows variation in accordance with variation in the intensity of the biasing current for the recording, and that correlation between these two variations differs as a function of the frequency of the recorded sound, as diagramatically shown in FIG. 1 of the accompanying drawings, which is a chart with the magnetic tape recording bias current intensity on abscissa and the reproduced sound output level on ordinate, giving two typical curves with respect to the sounds of 400 Hz and 10 kHz, respectively.
As is apparent, ratio of the reproduced sound output level variation to the recording bias current intensity variation is smaller in the case of comparatively low frequency sound than in the case of comparatively high frequency sound.
Thus, remarkable effect of variation in the intensity of the recording bias current over the frequency characteristic chart of the reproduced sound with a given tape-recording apparatus is seen in the higher frequency range, as illustrated in FIG. 2.
In order to realize optimal frequency characteristics with the widest flat range, it is essential to properly select the recording bias current intensity as will result in the same reproduced sound output level in the low frequency signal range (as may be represented for instance by 400 Hz) as in the higher frequency signal range (as may be represented for instance by 10 kHz). Such optimal biasing current intensity is designated in FIGS. 1 and 2 at P.sub.o.
Said chart of correlation between the recording bias current intensity and the reproduced sound level also differs or deflects considerably as the magnetic characteristics of the recording tape material differ, and it is also in the higher frequency range that such deflection of the characteristics is prominent.
For instance, FIG. 3 illustrates the comparison of the characteristic curves, similar to those shown in FIG. 1, thus for 400 Hz and 10 kHz, with respect to the tape using CrO.sub.2 as the magnetic material (shown in solid lines) and the tape generally referred to as "normal", using .gamma.-Fe.sub.2 O.sub.3 (in broken lines), and it is thus apparent that the said optimal biasing current intensity P.sub.o ' in the case of CrO.sub.2, to give the same reproduced sound output level both for 400 Hz and 10 kHz, is larger than the intensity P.sub.o of the same optimal nature in the case of .gamma.-Fe.sub.2 O.sub.3. In other words, the former material CrO.sub.2 requires stronger recording bias current for realizing the ideal sound reproduction frequency characteristics, with the widest flat range, than does the latter material .gamma.-Fe.sub.2 O.sub.3.
It is essential, therefore, to select the optimal recording bias current intensity in proper accordance with the magnetic characteristics of the particular tape material actually in use, for realizing said ideal sound reproduction frequency characteristics with the widest flat range, and in view that one must foresee to use various kinds of tapes, of the materials with magnetic characteristics different from one to another (for instance CrO.sub.2, FeCr, .gamma.-Fe.sub.2 O.sub.3 and so forth), in tape-recorders, especially those of casette-type, there have by now been in use those wherein the recording bias current is adjustable or variable in accordance with the kind of the magnetic tape actually used.
For instance, there have been tape-recorders with such circuitry for adjusting the recording bias current as may switch over the current intensity in a few pre-set steps or as may effect further precise readjustment by means of continuously manipulating a variable resistor after first selectively switching over the current also in a few steps, in accordance with the kind of the magnetic tape actually used.
As is apparent, it is not possible with the tape-recorders having the circuitry as described above, to always set the truly optimal recording bias current intensity for all the magnetic tapes with the magnetic characteristics different from one to another in a quite wide range, since the bias current intensity can only be switched over in a few steps or at best can further precisely be readjusted near each of said a few steps, and still more the mode of such manipulation as is required for actually manually adjusting the recording bias current intensity is very troublesome and time-consuming, even leading to the fear of failing at all to realize the optimal adjustment of the recording bias current, from erroneous switching operation or accidental improper manipulation, as may be apt to occur.